Modern continuous ion sources provide up to 109 ions per second in the case of Electrospray Ionisation (ESI), photoionisation (PI) and Atmospheric Pressure Chemical Ionisation (APCI) sources, up to 1010 ions per second in the case of Electron Ionisation (EI) sources and up to 1011 ions per second in the case of Inductively Coupled Plasma (ICP) ion sources. Desirably, for the highest sensitivity and speed of analysis, mass analysis of ions from these ion sources would be achieved, without allowing ions to go to waste, on a continuous basis.
Sequential scanning mass analyzers comprise a mass filter, which transmits ions with only a narrow range of mass to charge ratios at a given time. Hence, such analyzers waste any ions that are not of the mass to charge ratio being transmitted, since these ions are lost.
This means that existing mass spectrometers using such analyzers operate with a low duty cycle (typically between 0.1% and 10%) when performing quantitation analysis where multiple target compounds need to be analysed. The use of continuous ion sources, such as electrospray ionisation sources, may mean that ions are wasted when the analyser duty cycle is so low.
Nevertheless, sequential scanning mass analyzers offer improved linearity and dynamic range over the alternatives. Hence, it is desirable to improve the duty cycle of such mass spectrometers. Similar issues arise in relation to other types of ion spectrometer, such as ion mobility spectrometers or mass spectrometers, in which improvement in the duty cycle may be especially beneficial.